The long term objective of this research program is to delineate the B lymphocyte differentiation pathway. Initially, a panel of monoclonal antibodies will be used to define distinct B cell subsets. Distinct B cell subsets will be correlated with their immunological capacity to respond to different forms of antigens and mitogens. Various times after injecting members of distinct B cells subsets (of parental origin) into F1 recipients, spleens of the recipient will be removed and the cells of host origin eliminated. This procedure allows one to follow the fate of the injected (donor) cells with respect to marker expression as well as functional ability to respond to antigens and mitogens as they mature in a physiological environment. These transfer experiments will also be performed with the immediate precursor of B cells, pre-B cells. Results generated from such studies will reveal the differentiation potential of distinct B cell subsets and of pre-B cells, thus establishing a more detailed B lymphocyte differentiation pathway. A firmly established pathway of normal B lymphocyte differentiation should provide clues to understanding the nature of B cell disorders such as systemic lupus erythematosis.